powerpc/mpc8xxx: Add x4 DDR device support

On selected platforms, x4 DDR devices can be supported. Using x4 devices may
lower the performance, but generally they are available for higher density.

Tested on MT36JSF2G72PZ-1G9E1 RDIMM.

Signed-off-by: York Sun <yorksun@freescale.com>

arch/powerpc/include/asm/fsl_ddr_sdram.h

@@ -277,6 +277,7 @@ typedef struct memctl_options_s {
 	unsigned int mirrored_dimm;
 	unsigned int quad_rank_present;
 	unsigned int ap_en;	/* address parity enable for RDIMM */
+	unsigned int x4_en;	/* enable x4 devices */
 
 	/* Global Timing Parameters */
 	unsigned int cas_latency_override;